1. Field of the Invention
The present invention relates to a method for temporarily securing electronic devices on a printed circuit board during soldering. More particularly, it relates to a method for temporarily securing surface-mounting electronic devices onto a printed circuit board during soldering with a pressure-sensitive adhesive.
2. Description of the Prior Art
Recently, as electronic instruments have become compact, electronic devices used therein also have had to become more compact. As a consequence, surface mounting of electronic devices by directly securing electronic devices on the surface of a printed circuit board by means of soldering has been widely employed. Electronic devices adapted for surface mounting are called surface-mounting devices (hereinafter abbreviated as SMD's) and include flat-pack IC's and LSI's, as well as chip parts such as chip capacitors.
There are two methods for soldering SMD's to a printed circuit board: the reflow method and the dip method.
In the reflow method, a paste or cream solder (hereinafter referred to collectively as paste solder) is applied by printing to the areas to be soldered on the surface of a printed circuit board. Surface-mounting electronic devices (SMD's) are then positioned on the board and the board is heated in a reflow furnace to melt the paste solder and perform soldering of the ( SMD's to the board. Before the SMD's are mounted on the board, if necessary, an adhesive may be applied to the areas on the surface of the board on which SMD's are to be mounted in order to prevent the SMD's from dropping from the board during heating in the reflow furnace.
In the dip method, an adhesive is applied to the areas on the surface of a printed circuit board on which SMD's are to be mounted, and SMD's are mounted on the board through the adhesive and temporarily secured in position by the adhesive. The board is then processed to solder the SMD's to the board, e.g., using an automatic soldering machine which comprises a fluxer for applying a flux to the surface of the board, a preheater for preheating the board, a molten solder bath in which the board is dipped to deposit the molten solder around the terminals of the SMD's and the board, and a cooling unit for solidifying the molten solder deposited on the board.
Thermosetting adhesives and UV-curing adhesives have been used in the prior art to temporarily secure SMD's during soldering. The nature of adhesion with these adhesives is temporary before they are cured. However, the adhesive applied to the board is cured prior to or during soldering by heating in a suitable heater or in a molten solder bath or by irradiation with UV rays, thereby firmly fixing SMD's on the board in an undetachable manner, and soldering is performed under such conditions. Thus, SMD's are not temporarily secured on the printed circuit board during soldering, but are firmly fixed thereon.
Japanese Utility Model Publication No. 56-11438(1981) discloses a row of SMD's which comprises a plurality of SMD's detachably secured in a row on a tape by means of an adhesive comprising at least a thermosetting adhesive. The SMD's can be detached from the tape along with the adhesive and mounted onto a printed circuit board by the adhesive before soldering. The SMD's are firmly fixed on the board during soldering since the thermosetting adhesive is cured by heating prior to or during soldering.
Japanese Patent Application Kokai No. 64-69092(1989) describes a surface mounting device (SMD) which has an adherent or tacky thin layer on the seating plane thereof and which can be temporarily secured to a printed circuit board. The thin layer specifically disclosed in the example of that Japanese application is a thin layer which is impregnated with a thermosetting adhesive. Therefore, after the SMD's are mounted on a printed circuit board, they can be detached from the board while the adhesive is kept uncured before heating. However, they are heated immediately after mounting in order to thermoset the adhesive so that they are firmly fixed to the board during soldering. Thus, the nature of adhesion cannot be said to be temporary in a strict sense.
The above Japanese application discloses that the thin layer may be impregnated with a pressure-sensitive adhesive to impart tackiness to the layer. However, there is no specific disclosure concerning the pressure-sensitive adhesive. Most conventional pressure-sensitive adhesives are not considered to be effective at high temperatures above 200.degree. C. at which soldering is performed, and therefore it has not been attempted to use a pressure-sensitive adhesive to temporarily secure SMD's on a board during soldering.
The use of adhesives, and particularly UV-curable adhesives is costly since these adhesives themselves are expensive and a special, complicated UV irradiation apparatus is required to cure the adhesives. After a UV-curable adhesive is applied to a printed circuit board, the board must be passed through the UV irradiation apparatus, which is troublesome.
When a thermosetting or UV-curing adhesive is used to secure SMD's onto a printed circuit board, SMD's are firmly secured to the board before or during soldering. In one sense, this is advantageous in that SMD's are rarely detached from the board even if a considerable force is applied thereto. However, when SMD's are mounted out of position onto the board, the
adhesive firmly holds the mispositioned SMD's on the board, so solder is sometimes deposited in an undesirable manner. For example, the solder is deposited such that it does not connect one or more sets of terminals on the SMD's and on the board which should be connected by the solder, so that a proper circuit is not formed. Furthermore, solder may be deposited so as to connect an undesirable combination of terminals on the SMD's and on the board resulting in a short circuit.
Under the existing circumstances in which simplification of production lines for electronic instruments is required in order to decrease product costs, the use of a thermosetting or UV-curable adhesive to secure SMD's during soldering is not sufficient.